. Embryology. Embryology; Embryology; genealogy. 108 EMBRYOLOGY. Fig. 53. An amphibian embryo in a stage of transition from the neurula to the functional larva. Some structures still show the char- acteristics of fields, while others are differentiated in more detail. The head (right) is better developed than the rest of the body. Internally the eye and brain are undergoing structural differentiation. differentiation—that is, we assume that chemical differences exist between the various fields. One field is chemically different from another, although it is not visibly different. This is an inv
. Embryology. Embryology; Embryology; genealogy. 108 EMBRYOLOGY. Fig. 53. An amphibian embryo in a stage of transition from the neurula to the functional larva. Some structures still show the char- acteristics of fields, while others are differentiated in more detail. The head (right) is better developed than the rest of the body. Internally the eye and brain are undergoing structural differentiation. differentiation—that is, we assume that chemical differences exist between the various fields. One field is chemically different from another, although it is not visibly different. This is an invisible type of differentiation. Finally at fa, when the fields actually begin to exhibit structure, we may speak of struc- tural differentiation, which simply means a visible differentiation of the parts. There is still another way of looking at development. We may consider it in terms of the potencies of tissues. It will be recalled that potencies are simply abilities of the tissues to differentiate in various directions. Certainly in the early egg there are many potencies. We have outlined them earlier: When we get to the time fa, at which fields are present, the potencies become more restricted and fewer in number. And when we come to the stage of structural differentiation, we have reached the stage of having only single potencies. Thus we find a progressive restriction in what the tissues can do. Again a note of warning must be sounded. We pass beyond the experimental facts when we maintain that the parts of the fully developed embryo possess single potencies. Indeed the facts from studies on regeneration speak against this rigid restriction of potencies. Cells in the adult may still possess latent ability to differentiate into a new type of cell. Although the process of development has just been presented in three steps, bear in mind what was said earlier about stages. Steps and stages are purely arbitrary, and development is gradual and continuous. There is no sharp ti
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